Environmental Geology - - GEOL 406/506

Rock Strength and Behavior

As you might imagine, natural substances behave according to their internal
properties. They are either brittle or ductile, depending on the amount of stress
applied. The stress/strain graphs below illustrate the bahavior of materials
under stress.

THE ONLY WAY TO DETERMINE THE STRENGTH OF A GIVEN MATERIAL IS TO . . .

BREAK IT!

Elastic Deformation

A brittle substance fails (ruptures) after elastic deformation. An example
is a piece of window glass which will bend slightly, then break if enough stress
is applied. If stress is released before the glass breaks, it will return to
its original shape.

A
= substance with high yield strength, very little deformation prior to failure.

Ductile Deformation

Ductile deformation occurs when a substance is stressed to a point where it
begins to behave like a plastic. At this point the proportional elastic
limit has been exceeded and the substance begins to deform with less
and less applied stress. The ultimate strength of the material
is the highest point on the curve in the ductile region. The rupture
strength is usually lower than the ultimate strength and depends on
many factors, including the confining pressure and the amount of strain that
has occurred.

Most rocks have a very small region of elastic behavior and behave more like
a plastic with increased stress (pressure).

Permanent Strain

Permanent
strain occurs when the proportional elastic limit is exceeded and the substance
does not return to its original shape when the stress is released. The small
region in green illustrates the amount of strain that cannot be recovered once
the stress/strain curve has reached point "A". A piece of wire is
a good example of a substance that, when subjected to slight stress, will return
to its original shape, but, when the elastic limit is exceeded the wire is bent.

A piece of wire is not a good example of a natural substance
that has a finite rupture strength. Why?